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Surveying and driving tunnels must count among the most difficult of engineering projects. Not only are conditions of work underground unpleasant and dangerous, but to establish and maintain the required gradient and the required alignment through solid rock demand skills of a high order.

Tunnels could be of two basic kinds. One was the single-ended tunnel with only one mouth opening to the surface, the other end being underground in an aquifer as in the Persian qanat or in orebearing rock as in a mine adit. The other was the through tunnel for conducting water (or occasionally a road) through an obstruction such as a mountain ridge. This variety could be driven by two different methods.

The first, but certainly the second best, was the two-ended tunnel, where the depth of rock above was too great to sink shafts, except perhaps near each end for the purposes of alignment. The best known example is that built about 530 BC for the tyrant Polycrates by Eupalinus of Megara to bring water to the town of Samos. Its length was 1036 m, but it was not the longest of its kind.